An 82-year-old woman was hospitalized into the cardiac intensive care unit with complaints of a sharp pain behind the sternum, a sensation of lack of air, and weakness. Chest X-ray shows that the transverse size of the cardiac shadow is enlarged, the shape of the shadow is tri angular and has rounded cardiodiaphragmal corners. Cardiac contractions are of small amplitude and arrhythmic. These X-ray findings most likely correspond with: